The present invention relates generally to an apparatus and method for measuring properties of an earth formation traversed by a borehole, and more particularly, to an apparatus and method for determining a drilling mode to optimize formation evaluation measurements.
To make downhole measurements while a borehole is being drilled, measurement-while-drilling (MWD) and/or logging-while-drilling (LWD) systems are generally known which measure various useful parameters and characteristics such as the formation resistivity and the natural gamma ray emissions from the formations. Signals which are representative of these measurements made downhole are relayed to the surface with a mud pulse telemetry device that controls the mud flow, encoding information in pressure pulses inside the drill string. The pulses travel upward through the mud to the surface where they are detected and decoded so that the downhole measurements are available for observation and interpretation at the surface substantially in real time. As an alternative, it has also been found useful to provide a downhole computer with sufficient memory for temporarily storing these measurements until such time that the drill string is removed from the borehole.
U.S. Pat. Nos. 5,130,950 issued to Orban et al., 5,241,273 issued to Martin Luling, 5,017,778 issued to Peter D. Wraight, 5,148,407 issued to Haldorsen et al., 5,585,556 issued to Petersen et al., and 5,705,927 issued to Sezginer et al., describe MWD tools which employ nuclear magnetic resonance, sonic, seismic, nuclear, or electromagnetic measurements. The tools disclosed in the prior art have disadvantages which limit their utility in MWD and/or LWD applications. Sonic, resistivity, nuclear, electromagnetic, and seismic measurements are directly influenced by the drilling noise. For example, while acoustic energy generated at the surface is usually very large, the energy that must be detected at the drill bit can be very small due to geometrical spreading and attenuation of the acoustic waves in the subsurface formation. In many cases, the drilling noise is orders of magnitude larger than the acoustic wave energy propagating from the surface to the subsurface MWD detector. Also, MWD and LWD nuclear magnetic resonance measurements are directly influenced by the vertical and lateral motion of the tool. For example, due to the amount of time required to obtain T.sub.1 and T.sub.2 measurements, the formation properties may change during the measurement cycle. These aforementioned factors adversely affect MWD and/or LWD measurements.
In the presence of a noisy drilling environment, the prior art tools obtain nuclear magnetic resonance, sonic, electromagnetic, nuclear, and seismic measurements directly influenced by the vertical and lateral tool motion and the drilling noise. None of the MWD and/or LWD tools determine the drilling mode and accordingly modify the data acquisition sequence to optimize formation evaluation measurements.